1. Field of the Invention
The present invention relates to the field of data storage systems. In particular, the present invention relates a system and a method for determining a best pathway to requested data based on a metric, thereby improving device failure protection of an array of storage devices.
2. Description of the Related Art
A conventional array of storage devices typically has sufficient redundancy so that when a storage device fails, information contained in the failed storage device can be reconstructed from the remaining storage devices. See, for example, U.S. Pat. No. 5,579,475 to M. M. Blaum et al., entitled “Method and Means for Encoding and Rebuilding the Data Contents of Up to Two Unavailable DASDs in a DASD Array Using Simple Non-Recursive Diagonal and Row-Parity,” which discloses the operation of an array having distance D=3. See also, N. K. Ouchi, “Two-Level DASD Failure Recovery Method,” IBM Technical Disclosure Bulletin Vol. 36, 03 Mar. 1993, discloses the operation required for reconstructing data from an array with failures and having distance D=3.
Redundancy may also be used for improving performance. See, for example, E. J Schwabe et al., “Evaluating Approximately Balanced Parity-Declustered Data Layouts for Disk Arrays,” ACM 0-89791-813-4/96/05 1996, which disclose data layouts for efficient positioning of redundant information for performance. See also G. A. Alvarez et al., who, in “Tolerating Multiple Failures in RAID Architectures,” ACM 0-89791-901-7/97/0006 1997, disclose properties and construction of a general multiple-parity array using 8-bit finite fields, and L. Xu and J. Bruck, who, in “Improving the Performance of Data Servers Using Array Codes,” Paradise ETR027 (CalTech) 1998, describe the use of a maximum distance separation (MDS) code for improving system response.
Existing RAID (Redundant Array of Independent Disks) systems have multiple pathways, or routes, for reading requested data. Often, however, there are only a few available pathways, of which only one pathway is efficient. For example, a RAID 5 system provides two pathways for reading information. One pathway is by directly reading the sector containing the requested information. The second pathway is by reconstructing the data sector containing the requested information by reading the appropriate sector from each other storage unit in the RAID 5 array. As another example, an N storage unit array that is configured as a RAID 6 system can read a data sector directly or can read all other storage units, except one. Accordingly, there are N ways for RAID 6 to read a data sector. In both instances, there is one efficient pathway and one or more inefficient pathways.
Thus, when there are many pathways for obtaining requested data, for example, more pathways than the number of storage devices in the array, it is not trivially clear which pathway provides in the highest performance for a storage system. Consequently, what are needed are a system and a method for determining which pathway to select when a storage system uses a redundancy method having many pathways to requested data.